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Why do people get angry about
"Out of Control?"
Q. I've taken some courses and have
been starting some statistical work in my small company, and the
engineer who works with me just goes crazy when I use the term
"out of control."
On one occasion, he adjusted the knob
on a machine and I said, "Now your chart will show for sure
that you've made it go out of control."
He got so mad we
almost had a fistfight. What is the deal with this term "out
of control?"
A. The deal is this: The term
"out of control" is a hot button for many people! This
makes it an unfortunate term! It may have to do with the fact
that the engineering meaning of "control" is
practically the opposite of the statistical meaning of
"control."
Consider the following: If your engineer
friend was operating a toy 4-by-4 truck using a radio
transmitter, and he was turning the steering wheel this way and
that way, he would say "I have it under control."
If
the transmitter went dead and the truck went off due west, more
or less in a straight line except bumping a little on the gravel,
he'd say "Oh no, I've lost control."
Now compare the
statistical definition.
Statistical in-control means having
no identifiable changes acting on a process, just random variation around a
central tendency. Statistical in-control
is more analogous to the truck rumbling off due west with the transmitter
dead.
Statistical out-of-control means
identifiable changes are occurring to the average value or
variation-among-values of a process. Statistical
out-of-control is more analogous to the truck responding to
the transmitter and making figure-eights and turns.
Remember when
he turned the knob you said it would be out of
control? You were right.
Since the engineering and
statistical meaning of the terms are so different, many
practitioners use the terms "common cause variation"
and "special cause variation." Those terms cause less
trouble, and might spare you a black eye.
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